This invention relates to a novel multibeam electron gun having a cathode-grid subassembly and to a novel method for assembling that electron gun.
U.S. Pat. No. 4,298,818, issued to H. E. McCandless on Nov. 3, 1981, describes an electron gun for use in a multibeam cathode-ray tube. That gun includes at least two spaced successive electrodes held in position from a common support. Each electrode comprises a single metal plate having three beam-defining apertures therein, which apertures are so aligned as to permit the passage of three electron beams. The sizes and shapes of the electron beams are determined, in part, by the sizes, shapes and alignments of the apertures.
When there are three or more beam-defining apertures in each of two spaced single-plate electrodes, it is the practice to align the apertures of the electrodes, either optically or mechanically, from two of the beam-defining apertures of each of the electrodes. While the positioning of the apertures in each electrode are precisely prescribed, nevertheless there are necessary manufacturing tolerances present in the fabrication of these electrodes and in the alignment pins which maintain the alignment of the beam-defining apertures during a brazing operation.
U.S. Pat. No. 4,500,808 issued to H. E. McCandless on Feb. 19, 1985, describes an electron gun and method of assembly in which one of the electrodes is a composite structure comprising a support member and a plurality of plate members. The plate members, each of which has a single beam-defining second aperture, are separately aligned to one of the first apertures in the other electrode by means of alignment pins extending through each of the first and second beam-defining apertures. The plate members are brazed to the support member to maintain the precise alignment between the first and second apertures. The manufacturing cost of such a structure is relatively high, since a larger number of accurately dimensioned electron gun components are required than in the patented structure described in U.S. Pat. No. 4,298,818. Furthermore, it is desirable to avoid using alignment pins through the beam-defining apertures of the electron gun electrodes, since the pins may distort or scratch the material surrounding the apertures, thereby causing uncontrolled variations in electron beam size and shape.
A copending U.S. Pat. application Ser. No. 643,175, filed on Aug. 22, 1984, by H. E. McCandless et al., entitled, "MULTIBEAM ELECTRON GUN HAVING A CATHODE-GRID SUBASSEMBLY AND METHOD OF ASSEMBLING THE SAME", discloses a structure and method for indirectly aligning the beam-defining apertures of an electron gun by means of a pair of substantially identical alignment apertures. The screen grid electrode in the structure described in the copending application is also a composite structure; however, the number of screen grid electrode components have been reduced from four, as described in U.S. Pat. No. 4,500,808 to two.
The need continues to exist for an even simplier grid structure that permits accurate alignment of the beam-defining apertures in the control grid electrode and those in the screen grid electrode.